A conventional method of treating a patient suffering from cardiac disease or some other cardiac disorder involves monitoring the performance of the patient's heart and applying treatment, as necessary, based on the monitored performance. For example, measurement of cardiac output provides an indication as to how effectively the heart is pumping blood. Thus, a decrease in cardiac output may indicate that the patient's condition is worsening and, consequently, that new therapy should be administered to the patient or the at the patient's current therapy should be modified.
Conventionally, cardiac output may be approximated based on analysis of other cardiac parameters. One such parameter is stroke volume—the volume of a blood ejected from a ventricular chamber upon contraction of the ventricle. Stroke volume may be estimated based on analysis of a sensed thoracic impedance signal. Here, a sensing mechanism may measure impedance across a portion of the thoracic cavity of a patient over a given time period. The time derivative of the thoracic impedance is known as cardiogenic impedance. Stroke volume information may be derived from the cardiogenic impedance. For example, stroke volume information may be derived from the maximum of the cardiogenic impedance signal and from ventricle ejection time derived from the cardiogenic impedance signal.
Typically, the thoracic impedance signal includes one or more signal components in addition to a cardiogenic component. For example, air moving into and out of the lungs may affect the measured thoracic impedance. Thus, a thoracic impedance signal may contain a respiratory component that corresponds to the breathing pattern of the patient. A thoracic impedance signal also may include signal component resulting from motion of the patient.
In practice, it may be difficult to separate a cardiac-related component (from which cardiogenic impedance is derived) from other components of the thoracic impedance signal due to the relatively similar frequency characteristics of these components. For example, the bandwidth of the cardiac-related component may be on the order of 0.8-20 Hz. The bandwidth of the respiratory component may be on the order of 0.04-2 Hz. The bandwidth of a motion-related component may be on the order of 0.1-10 Hz. Given the relative similarity of these frequency ranges, conventional linear techniques may not provide a sufficiently effective mechanism for separating the thoracic impedance signal components.
Other techniques such as cessation of breathing, ensemble averaging and adaptive digital filtering have been proposed for removing a respiratory component from a thoracic impedance signal. However, each of these techniques may have one or more drawbacks. For example, for a patient with cardiac-related problems it may not be practical or advisable to have the patient hold his or her breath every time a thoracic impedance measurement is taken.
Ensemble averaging may be used to suppress beat-to-beat variations and transients in the derived cardiogenic component signal. This process, however, tends to suppress the distinctive A/B/X notch in the cardiogenic impedance signal. This, in turn, may adversely affect derivation of the stroke volume information because accurate calculation of stroke volume may depend on accurate acquisition of the A/B/X notch information.
A high pass infinite impulse response digital filter may be used to filter out a respiratory component. Such a filter may incorporate a varying cutoff frequency to compensate for changes in the heart rate of a patient. In practice, however, such a technique may distort signals in hearth rhythm transition. For example, the filter may not have advance knowledge as to when a patient's activity level will change between exercise and resting states. As a result, improper filtering may be employed during these transitions.